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Updated: June 3, 2025
Care must be exercised by the beginner, however, not to confuse sensations with the nervous impulses, on the one hand, or with secondary mental effects, such as emotion or imagination, on the other. Sensations are properly regarded as the first conscious effects of the afferent impulses and as the beginning stage in the series of mental processes that may take place on account of them.
For applied to our previous example of a flash of light, the incoming message corresponds to the sensuous report of the flash, the outgoing message to the closure of the eye, and the unfortunate central office girl has disappeared. The afferent nerve reports directly to the efferent, without passing the message through consciousness.
It is the latter or afferent function with which we are now concerned. The range of our sense-perceptions puts us momentarily in relations with the material world, or rather, with a certain portion of it. For we by no means sense all that is sensible, and, as I have already indicated, our sense impressions are often delusive.
This lack of blood and sensation of cold are at once telegraphed over the afferent nerves to headquarters in the brain, and from there the command goes forth to the nerve centers regulating the circulation: "Send blood into the surface!"
"Why did God punish not only the guilty pair, but also the innocent unborn generations?" Master was more amused by my vehemence than my ignorance. "GENESIS is deeply symbolic, and cannot be grasped by a literal interpretation," he explained. "Its 'tree of life' is the human body. The spinal cord is like an upturned tree, with man's hair as its roots, and afferent and efferent nerves as branches.
MacDonald offers as a physiologic explanation of this case that probably the impression produced forty-two years before implicated the right brachial plexus and the afferent branches of the pneumogastric, and to some degree the vomiting center in the medulla; hence, when the atmosphere was highly charged with electricity the structures affected became more readily impressed.
The exaggerated afferent impulses reaching the brain as a result of trauma, inhibit the action of the nuclei in the region of the fourth ventricle and cerebellum which maintain the muscular tone, with the result that the muscular tone is diminished and there is a marked fall in the arterial blood pressure.
McDougall that the best test of the instincts that we have is to be found in the specific emotions. He believes that every instinctive process consists of an afferent part or message, a central part, and an efferent part or discharge. At its two ends the process is highly plastic. Message and discharge, to which thought and will correspond, are modified in their type as experience matures.
'When the eyelids wink at a flash of light, or a threatened blow, a reflex action takes place, in which the afferent nerves are the optic, the efferent, the facial. When a bad smell causes a grimace, there is a reflex action through the same motor nerve, while the olfactory nerves constitute the afferent channels.
This circuit of communication from the cell over the afferent nerves to the nerve centers in the brain or spinal cord, and from these centers over the efferent nerves back to the cell or to other cells is called the reflex arc. Let us use an illustration: Suppose the fingers come in close contact with a hot iron. The cells in the finger tips experience a sensation of burning pain.
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